Various dispensing systems have been used for applying molten thermoplastic material, such as hot melt adhesives, onto a moving or stationary substrate. In the production of liquid containers, such as juice cartons, hot melt adhesive systems have been developed to apply hot melt adhesive to the mounting flange of the carton cap to bond and seal the non-removable portion of the cap to the carton. In this application, the hot melt adhesive must be applied in a pattern that includes a continuous, i.e., unbroken, outer perimeter to hermetically seal the non-removable portion of the cap to prevent contamination of juice or other liquid within the carton or container when the cap is closed and to prevent spillage when the cap is open and the liquid is poured from the container. The pattern of the dispensed adhesive may have a round or other pattern, depending upon the shape of the cap.
Typically, the dispenser is mounted above a moving conveyor or other device carrying the caps, with the hot melt adhesive dispensed onto the mounting flanges of the caps. One conventional system has utilized a dispenser having a single orifice, with the dispenser mounted on a dedicated automation device capable of moving the dispenser as required to create the desired adhesive pattern on the flange as it moves past the dispenser. The production line speed may be very high, for example, the dispenser may be required to dispense adhesive onto the cap flanges at a rate of 2/sec. As may be appreciated, complex and expensive dedicated automation devices, such as robots, are required to move the dispenser fast enough to achieve the desired pattern of applied adhesive in view of the high production line speed. To prevent an undesirable and costly reduction in line speed, conventional systems have used multiple dispensers, each mounted on separate robot heads. This adds cost to the dispensing system.
Other challenges associated with hot melt adhesive dispensing systems include the prevention of discharge orifice clogging that may potentially occur due to the presence of charred particles of adhesive or other contaminants within the dispensing system and the creation of acceptable line quality. The deposition of hot melt adhesive must be the correct amount within fairly tight tolerances in many applications including the “carton cap” application discussed previously. If insufficient adhesive is deposited, the carton cap may leak. If too much adhesive is deposited, there may be an undesirable “squeeze-out” of material around the edges of the cap. As part of the design process of sizing discharge orifices, the following tradeoffs must be assessed. For a given amount of adhesive at a particular application pressure, smaller orifices result in higher velocities which can contribute to better “cutoff”. This helps eliminate any undesirable “stringing” of the material that may otherwise occur and adversely affect the quality of the pattern of dispensed adhesive. However, relatively smaller orifices are more susceptible to clogging as a result of any charred particles or other contaminants that may be flowing through the system. Relatively larger orifices help minimize the chance of orifice clogging, but the challenge with relatively larger orifices is to maintain sufficiently high pressure to maintain an acceptable cutoff velocity while keeping the pressure low enough to avoid the deposition of excess adhesive.